JP2006161668A - Exhaust emission control system and desulfurization control method for exhaust emission control system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device and a desulfurization control method for the exhaust emission control system suppressing H<SB>2</SB>S generation quantity and increasing ratio SO<SB>2</SB>in desulfurization control for sulfur poisoning regeneration and maintaining catalyst temperature during desulfurization control in the exhaust emission control system using NOx storage reduction type catalyst. <P>SOLUTION: In the exhaust emission control system provided with NOx storage reduction type catalyst, control to make air fuel ratio in an inlet side of the NOx storage reduction type catalyst to a rich side and control to make the same to lean side are alternately repeated in desulfurization control for regenerating sulfur poisoning of the NOx storage reduction type catalyst. Target air fuel ratio in the inlet side is set to 0.85-0.95 in excess air ratio conversion in control to rich side, and to 1.05-1.15 in excess air ratio conversion in control to lean side. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a desulfurization control method and an exhaust gas purification system for an exhaust gas purification system including a NOx occlusion reduction type catalyst that reduces and purifies nitrogen oxides (NOx) in exhaust gas of an internal combustion engine.

  Various studies and proposals have been made on NOx catalysts for reducing and removing NOx from internal combustion engines such as diesel engines and some gasoline engines and exhaust gases from various combustion devices. One of them is a NOx occlusion reduction type catalyst as a NOx reduction catalyst for diesel engines, which can effectively purify NOx in exhaust gas.

  This NOx occlusion reduction type catalyst basically has a noble metal such as platinum (Pt) or palladium (Pd) that promotes an oxidation / reduction reaction and an alkaline earth such as barium (Ba) on a catalyst carrier such as alumina. It is a catalyst carrying a NOx occlusion material (NOx occlusion material) having a function of occluding and releasing NOx formed of a similar metal.

This NOx occlusion reduction type catalyst has a nitrogen (NO) concentration in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean (oxygen-rich) and oxygen (O ₂ ) is present in the atmosphere. Is oxidized by noble metals into nitrogen dioxide (NO ₂ ), and this NO ₂ is accumulated as nitrate (Ba ₂ NO ₄ etc.) in the NOx storage material.

Further, when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or a rich (low oxygen concentration) state and oxygen is not present in the atmosphere, the NOx storage material such as Ba is combined with carbon monoxide (CO). NO ₂ is decomposed and released from the nitrate, and this released NO ₂ is reduced by unburned hydrocarbons (HC), CO, etc. contained in the exhaust gas by the ternary function of noble metals, and nitrogen (N ₂ ) Thus, various components in the exhaust gas are released into the atmosphere as harmless substances such as carbon dioxide (CO ₂ ), water (H ₂ O), and N ₂ .

  Therefore, in an exhaust gas purification system equipped with a NOx occlusion reduction type catalyst, when the NOx occlusion capacity becomes close to saturation, the air-fuel ratio of the exhaust gas is made rich, and the oxygen concentration of the inflowing exhaust gas is reduced, for recovering the NOx occlusion capacity NOx absorbed is released by performing rich control of NOx, and NOx regeneration operation is performed in which the released NOx is reduced by a noble metal catalyst.

However, this NOx occlusion reduction type catalyst has a problem of performance deterioration due to sulfur poisoning. In other words, sulfur (sulfur) contained in the fuel becomes sulfur dioxide (SO ₂ ) by combustion and is stored in the storage material in the same manner as NO _2, and then sulfate such as barium sulfate (Ba ₂ SO ₄ ) is added. As a result, a phenomenon occurs in which the NO ₂ storage capacity decreases.

Therefore, it is necessary to create an environment where sulfates are easily decomposed in a timely manner and perform desulfurization control. One of the conditions necessary for this desulfurization is a high temperature of 600 ° C. or higher, although there are some differences depending on the catalyst. As for the oxygen concentration, if O ₂ is present, the reducing agent (HC, CO) uses O ₂ for oxidation, so that the decomposition of the sulfate (Ba ₂ SO ₄ ) is not accelerated and desulfurization does not occur.

Meanwhile, since the O ₂ is discharged becomes harmful gases altogether nonexistent when sulfur is combined with hydrogen (H ₂₎ hydrogen sulfide (H ₂ S), is required to release as again oxidizing the H ₂ S SO ₂ is there. In addition, since the heat generated by the oxidation reaction is small, the catalyst temperature is lowered during the desulfurization, so that an atmospheric condition containing a trace amount of O ₂ necessary for the oxidation of H ₂ S is required.

Therefore, maintaining the air-fuel ratio of the catalyst inlet side to the rich state than the stoichiometric air-fuel ratio, since the oxygen is insufficient, reacts with HC of released SO ₂ with a reducing agent, H ₂ to dissipate the stench S There is a problem that the occurrence rate of the is increased.

On the other hand, when the oxygen concentration at the catalyst outlet is fixed in the stoichiometric state, as shown in the right end A of FIG. 4, H ₂ S can be reduced, but the desulfurization amount is reduced to 1/5 or less. For this reason, the desulfurization control time for the sulfur purge to increase the catalyst temperature to about 700 ° C. becomes longer, which causes a problem of cost deterioration and catalyst deterioration.

  In addition, regarding desulfurization control for sulfur poisoning regeneration (recovery) of the NOx catalyst, in order to improve the efficiency of SOx release from the SOx absorbent (NOx absorbent), the air-fuel ratio of the exhaust gas at the SOx absorbent outlet is The air-fuel ratio of the exhaust gas flowing into the SOx absorbent is controlled so as to be the stoichiometric air-fuel ratio (stoichiometric), and in this air-fuel ratio control, the air-fuel ratio of the exhaust gas first flowing into the SOx absorbent is controlled to be rich, Thereafter, there has been proposed an exhaust purification device for an internal combustion engine that controls the air-fuel ratio of exhaust gas that gradually flows into the stoichiometric air-fuel ratio (see, for example, Patent Document 1).

However, as in this exhaust gas purification apparatus for an internal combustion engine, the excess air ratio λ on the catalyst inlet side is not more than 1.0 so that the air-fuel ratio of the exhaust gas at the SOx absorbent outlet becomes the stoichiometric air-fuel ratio (stoichiometric). If the state is maintained, oxygen is insufficient, so that the released SO ₂ reacts with the reducing agent HC to generate H ₂ S.

  In the desulfurization control, in order to prevent an over-rich value, at the time of SOx poisoning recovery, the ECU (control device) reduces the oxygen concentration in the exhaust gas flowing into the filter in a spike manner with a relatively short cycle. An exhaust emission control device for an internal combustion engine that executes fuel addition control (so-called rich spike control) has been proposed (see, for example, Patent Document 2).

However, this exhaust purification device does not cope with the change from SO ₂ to H ₂ S due to lack of oxygen. Further, practically, a specific numerical value of the rich and lean air-fuel ratio and a rich period and a lean time are required, but a specific combination thereof is not mentioned.
JP 2000-170525 A JP 2003-336518 A (page 5)

The present invention has been made to solve the above-described problems, and an object of the present invention is to recover sulfur poisoning in an exhaust gas purification system using a NOx occlusion reduction type catalyst for purification of NOx in exhaust gas. The desulfurization control method and exhaust gas of an exhaust gas purification system that can suppress the amount of H ₂ S generated in the desulfurization control and increase the SO ₂ ratio and can maintain the catalyst temperature during the desulfurization control It is to provide a gas purification system.

  The desulfurization control method of the exhaust gas purification system for achieving the above object is to store NOx when the air-fuel ratio of the exhaust gas is lean and store NOx stored when it is rich. In an exhaust gas purification system including a NOx occlusion reduction catalyst that releases and reduces, an air-fuel ratio on the inlet side of the NOx occlusion reduction catalyst is controlled by desulfurization to recover sulfur poisoning of the NOx occlusion reduction catalyst. In the rich side and the lean side control are alternately repeated, and in the control to set the target air-fuel ratio on the inlet side to the rich side, 0.85 to 0.95 in terms of excess air ratio. In the control on the lean side, the air excess ratio is converted to 1.05 to 1.15.

  Here, the air-fuel ratio state of the exhaust gas does not necessarily mean the state of the air-fuel ratio in the cylinder, but the amount of air and the amount of fuel supplied to the exhaust gas flowing into the NOx storage reduction catalyst (cylinder) (Including the amount burned inside).

In the desulfurization control for sulfur poisoning recovery (regeneration, sulfur purge) of the NOx storage reduction catalyst by this method of controlling the air-fuel ratio on the catalyst inlet side, the air-fuel ratio at the inlet of the NOx storage reduction catalyst is set to the excess air ratio. Converted periodically between 0.85 and 0.95, preferably 0.9 rich state and 1.05 to 1.15, preferably 1.1 lean state. As a result, a sufficient amount of reducing agent for raising the temperature of the exhaust gas can be secured, and a necessary amount of oxygen for oxidizing H ₂ S to SO ₂ can be provided. These specific numerical values are derived from experiments.

That is, by temporarily setting the air-fuel ratio at the inlet of the NOx storage reduction catalyst to a lean state of 1.05 to 1.15, the exhaust gas temperature rise and the oxygen atmosphere become appropriate, and H ₂ S is oxidized. it is possible to supply just the right amount of O ₂ that only the sO ₂ Te, can be reliably oxidize H ₂ S to sO _2. Accordingly, it is possible to promote desulfurization with SO ₂ and maintain the temperature during the desulfurization control by the heat generated by this oxidation. Thereby, while suppressing the generation amount of H ₂ S and increasing the ratio of S ₂ O, the amount of generated SO ₂ can be secured, the sulfur purge can be completed early, and the desulfurization control can be completed in a short time.

  Further, in the above desulfurization control method for the exhaust gas purification system, the target air-fuel ratio on the inlet side is feedback-controlled so that the air-fuel ratio on the outlet side of the NOx storage reduction catalyst becomes a stoichiometric state during the desulfurization control.

In other words, the value of the O ₂ sensor on the catalyst outlet side (rear stream side) is always less than or equal to the stoichiometric value, and if it is shallow and rich, the stoichiometric air / fuel ratio is maintained as a target and the air / fuel ratio on the inlet side or rich / lean is maintained. The target value of the interval is feedback controlled. As a result, after the middle stage of the catalyst, it always becomes equal to or lower than the stoichiometric air-fuel ratio, so that desulfurization is possible.

  Further, in the above-described desulfurization control method of the exhaust gas purification system, the time ratio between the rich side control and the lean side control that is repeatedly performed in the desulfurization control is rich side control: lean side control = 5: 2 to 4: 3. And By these controls, the ratio of the amount of reducing agent and the amount of oxygen suitable for desulfurization can be made appropriate. These specific numerical values are derived from experiments.

  An exhaust gas purification system for achieving the above object occludes NOx when the air-fuel ratio of the exhaust gas is in a lean state and releases NOx that has been occluded in a rich state. In the exhaust gas purification system comprising a NOx occlusion reduction type catalyst to be reduced together with a catalyst regeneration control device for performing desulfurization control for recovering sulfur poisoning of the NOx occlusion reduction type catalyst, the desulfurization control means comprises the NOx occlusion control means. The control to bring the air-fuel ratio on the inlet side of the reduction catalyst to the rich side and the control to make it lean is alternately repeated, and the desulfurization control means controls the target air-fuel ratio on the inlet side to the rich side. Then, it is set to 0.85 to 0.95 in terms of excess air ratio, and in the control on the lean side, it is configured to be 1.05 to 1.15 in terms of excess air ratio.

  In the exhaust gas purification system, the desulfurization control unit performs feedback control so that the air-fuel ratio on the outlet side of the NOx storage reduction catalyst becomes a stoichiometric (theoretical air-fuel ratio) state.

  In the above exhaust gas purification system, the desulfurization control means repeats the desulfurization control, and the time ratio between the rich side control and the lean side control is rich side control: lean side control = 5: 2-4. : 3.

According to the exhaust gas purification system desulfurization control method and the exhaust gas purification system according to the present invention, the air-fuel ratio at the inlet of the NOx storage reduction catalyst is in a rich state of 0.85 to 0.95 in terms of excess air ratio, and Since the feedback control is performed so as to be stoichiometrically downstream of the NOx occlusion reduction type catalyst periodically between the lean states of 05 to 1.15, a sufficient reducing agent for raising the exhaust gas temperature can be secured, A necessary amount of oxygen for oxidizing H ₂ S to SO ₂ can be provided.

In other words, H ₂ S can be reliably oxidized to SO ₂ by intermittently repeating the lean state without continuing to maintain the air-fuel ratio on the inlet side of the NOx storage reduction catalyst in a rich state. It is possible to maintain a high temperature for desulfurization.

This makes the oxygen atmosphere and exhaust gas temperature appropriate, suppresses the generation amount of H ₂ S and increases the proportion of SO ₂ , secures the generated desulfurization amount, completes the sulfur purge at an early stage, and controls the desulfurization. Can be completed in a short time.

  Hereinafter, an exhaust gas purification system desulfurization control method and an exhaust gas purification system according to an embodiment of the present invention will be described with reference to the drawings. Here, the air-fuel ratio state of the exhaust gas does not necessarily mean the state of the air-fuel ratio in the cylinder, but the amount of air and the amount of fuel supplied to the exhaust gas flowing into the NOx storage reduction catalyst (cylinder) (Including the amount burned inside).

  FIG. 1 shows a configuration of an exhaust gas purification system 1 according to an embodiment of the present invention. In this exhaust gas purification system 1, an exhaust gas purification device 10 having an oxidation catalyst 11a, a DPF 11b, and a NOx occlusion reduction type catalyst 11c from the upstream side is disposed in an exhaust passage 4 of an engine (internal combustion engine) E.

  The oxidation catalyst 11a is formed of a monolith catalyst having a large number of polygonal cells formed of a cordierite, SiC, or stainless steel structural material. On the inner wall of the cell, there is a catalyst coat layer having a large surface area, and a catalytic metal such as platinum or vanadium is supported on the large surface to generate a catalyst function.

  The DPF 11b can be formed of a monolith honeycomb wall flow type filter or the like in which the inlets and outlets of the porous ceramic honeycomb channels are alternately plugged, and collects PM (microparticles) in the exhaust gas. The DPF 11b may carry an oxidation catalyst or a PM oxidation catalyst in order to promote the combustion removal of PM.

  The NOx occlusion reduction type catalyst 11c is formed of a monolithic catalyst, and a catalyst coat layer is provided on a carrier such as aluminum oxide or titanium oxide, and a catalyst metal such as platinum (Pt) (Pd) is provided on the catalyst coat layer. A NOx occlusion material (NOx occlusion material) such as barium (Ba) is supported.

  In the NOx occlusion reduction type catalyst 11c, when the oxygen concentration is in an exhaust gas state (lean air-fuel ratio state), the NOx occlusion material stores NOx in the exhaust gas, thereby purifying NOx in the exhaust gas, In the exhaust gas state where the oxygen concentration is low or zero, the stored NOx is released and the released NOx is reduced by the catalytic action of the catalytic metal, thereby preventing NOx from flowing out into the atmosphere.

An upstream air excess rate sensor (λ sensor) 13 and a downstream oxygen concentration sensor (O ₂ sensor) 14 are disposed upstream and downstream of the NOx storage reduction catalyst 11c, that is, before and after.

  Further, an upstream temperature sensor 15 and a downstream temperature sensor 16 for determining the temperature of the NOx storage reduction catalyst 11c are arranged on the upstream side and the downstream side of the NOx storage reduction catalyst 11c, that is, on the front and rear sides, respectively.

  Further, an HC supply valve (fuel injector) 12 for supplying hydrocarbon (HC) as a reducing agent for NOx is provided in the exhaust passage 4 upstream of the exhaust gas purification device 10. The HC supply valve 12 directly injects hydrocarbons (HC) such as light oil as engine fuel from a fuel tank (not shown) into the exhaust passage 4 to adjust the air-fuel ratio of the exhaust gas to a rich state or a stoichiometric state (theoretical air). (Fuel ratio state). In addition, when the same air-fuel ratio control is performed by post-injection in the fuel injection in the cylinder of the engine E, the arrangement of the HC supply valve 12 can be omitted.

A control device (ECU: engine control unit) 20 that performs overall control of the operation of the engine E and also performs recovery control of the NOx purification ability of the NOx storage reduction catalyst 11c is provided. Detection values from the upstream λ sensor 13, the downstream O ₂ sensor 14, the upstream and downstream temperature sensors 15, 16, and the like are input to the control device 20, and the EGR valve 6 and the fuel of the engine E are transmitted from the control device 20. A signal for controlling the fuel injection valve 8 and the intake throttle valve (intake throttle valve) 9 of the common rail electronically controlled fuel injection device for injection is output.

  In this exhaust gas purification system 1, the air A passes through a mass air flow sensor (MAF sensor) 17 in the intake passage 2 and a compressor 3 a of the turbocharger 3, and the amount of the air A is adjusted by the intake throttle valve 9 to be taken into the intake air. It enters the cylinder from the manifold 2a. The exhaust gas G generated in the cylinder exits from the exhaust manifold 4a to the exhaust passage 4 to drive the turbine 3b of the turbocharger 3 and passes through the exhaust gas purification device 10 to become purified exhaust gas Gc. Then, it is discharged into the atmosphere through a silencer (not shown). A part of the exhaust gas G passes through the EGR cooler 7 of the EGR passage 5 as EGR gas Ge, and the amount thereof is adjusted by the EGR valve 6 and recirculated to the intake manifold 2a.

  A control device of the exhaust gas purification system 1 is incorporated in the control device 20 of the engine E, and controls the exhaust gas purification system 1 in parallel with the operation control of the engine E. The control device of the exhaust gas purification system 1 recovers the PM regeneration control for removing PM of the DPF 11b, the NOx regeneration control for recovering the NOx storage capacity of the NOx storage reduction catalyst 11c, and the sulfur poisoning of the NOx storage reduction catalyst 11c. Control of exhaust gas purification system such as desulfurization control.

  This PM regeneration control is a control that raises the exhaust gas temperature and oxidizes and removes the PM collected in the DPF 11b when the accumulated amount of PM in the DPF 11b increases and the clogged state deteriorates. .

  Further, the NOx regeneration control calculates the NOx emission amount ΔNOx per unit time from the operating state of the engine, and starts regeneration when the NOx accumulated value ΣNOx obtained by accumulating the calculated amount exceeds a predetermined determination value Cn, or When the NOx purification rate is calculated from the NOx concentration detected by the NOx concentration sensor (not shown) arranged upstream and downstream of the NOx storage reduction catalyst 11c, and this NOx purification rate becomes lower than a predetermined judgment value The regeneration of the NOx catalyst is started.

  Then, the air-fuel ratio of the exhaust gas is controlled to a stoichiometric air-fuel ratio (theoretical air-fuel ratio) or a rich state. In this control, the EGR valve 6 is controlled to increase the EGR amount or the intake throttle valve 9 is controlled. The air-fuel ratio is reduced by adding fuel into the exhaust gas by reducing the intake air amount by reducing the air-fuel ratio of the exhaust gas or by post-injection in the exhaust pipe injection or in-cylinder injection.

  By these controls, the exhaust gas is brought into a predetermined target air-fuel ratio state, and in a predetermined temperature range (approximately 200 ° C. to 600 ° C. depending on the catalyst), so that the NOx occlusion capability, that is, the NOx purification capability is improved. It recovers and the NOx catalyst is regenerated.

  And in this invention, desulfurization control is performed as follows.

  In this desulfurization control, the sulfur (sulfur) accumulation amount is integrated, and when the sulfur accumulation amount exceeds a predetermined determination value, the sulfur desulfurization (sulfur purge) control is started on the assumption that sulfur has accumulated until the NOx storage capacity decreases. In this desulfurization control, sulfates differ depending on the catalyst, but since they are not decomposed and released unless the rich conditions of about 600 ° C to 700 ° C are reached, from the viewpoint of efficient operation of energy, prior to this desulfurization control, PM regeneration control of the DPF 11b is performed to raise the exhaust temperature due to PM combustion and raise the temperature of the NOx storage reduction catalyst 11c.

  As shown in FIG. 2, the desulfurization control method of this exhaust gas purification system controls the opening of the intake throttle 9 and the EGR valve 6 so that the amount of intake air, EGR gas, etc. is constant, and post-injection or High temperature rich atmosphere is created by exhaust pipe injection. Furthermore, the control is performed so that the air-fuel ratio on the inlet side of the NOx occlusion reduction type catalyst 11c is 0.85 to 0.95, preferably 0.9, in terms of excess air ratio (λ), and in terms of excess air ratio. 1.05 to 1.15, preferably 1.1 to the lean side is alternately repeated.

  In this desulfurization control, the time ratio between the rich side control and the lean side control is set to rich side control: lean side control (R: L) = 5: 2 to 4: 3. It should be noted that the time for one cycle of the control on the rich side and the control on the lean side is, for example, the control time on the rich side is about 3 s to 5 s, and the control time on the lean side is 2 s. The time from the start to the end of the desulfurization control for repeating this cycle is about 3 min.

By controlling the air-fuel ratio on the catalyst inlet side, a sufficient amount of reducing agent for raising the exhaust gas temperature can be ensured, and a necessary amount of oxygen for oxidizing H ₂ S to SO ₂ can be provided.

Then, the air-fuel ratio on the inlet side is made to be in the lean state repeatedly without maintaining the rich state, thereby making the exhaust gas temperature rise and the oxygen atmosphere appropriate, and oxidizing H ₂ S to SO ₂ . Only O ₂ can be supplied.

As a result, H ₂ S can be reliably oxidized to SO ₂ , H ₂ S malodor can be prevented, desulfurization with SO ₂ can be promoted, and the temperature during desulfurization control can be maintained by the heat generated by this oxidation. It becomes. Thereby, by suppressing the generation of H ₂ S increasing the proportion of SO _2, to ensure the amount of generated SO _2, it can be terminated early by completing the sulfur purge desulfurization control in a short time.

  Further, in this desulfurization control method, the target air-fuel ratio on the inlet side is feedback-controlled so that the air-fuel ratio on the outlet side of the NOx storage reduction catalyst 11c is in a stoichiometric state during the desulfurization control, and the air-fuel ratio on the inlet side is also adjusted. Calculated from the intake air intake amount and the load (fuel injection amount).

In other words, the value of the downstream O ₂ sensor 14 on the catalyst outlet side (rear stream side) is always equal to or lower than the stoichiometric value, and the target value of the inlet air / fuel ratio is feedback-controlled so that the stoichiometric air / fuel ratio becomes the stoichiometric air / fuel ratio when shallow and rich. To do. As a result, it is possible to make the desulfurization possible by always setting the middle and subsequent stages of the catalyst below the stoichiometric air-fuel ratio.

  FIG. 3 shows an example of this control flow. This desulfurization control flow is repeatedly called from the control flow of the exhaust gas purification system 1 and returns as it is if desulfurization regeneration control is not necessary. When desulfurization regeneration control is necessary, the desulfurization control flow is performed and completed. It is illustrated as what returns.

  In this desulfurization control flow, it is determined in step S11 whether or not it is a desulfurization regeneration timing. If it is determined in step S11 that it is not the desulfurization regeneration time, the process returns. If it is determined that it is the desulfurization regeneration time, the process goes to step S12 to start the desulfurization regeneration control. In addition, a timer for the desulfurization time ts is started and measurement of the elapsed time ts of the desulfurization regeneration control is started.

  Then, in the next step S13, desulfurization rich side control is performed for setting the rich state for a predetermined time tr0. In the next step S14, the desulfurization time ts is checked to determine whether or not the desulfurization regeneration control is finished. When the desulfurization time ts exceeds the predetermined desulfurization completion time ts0, desulfurization completion is performed in step S18 as desulfurization completion, and the process returns.

  If the desulfurization time ts does not exceed the predetermined desulfurization completion time ts0, the desulfurization lean side control is performed in step S15 so that the lean state is maintained for the predetermined time tl0. In step S16 after step S15, the catalyst outlet side λ (excess air ratio) or catalyst outlet oxygen concentration during desulfurization regeneration control is checked. If the catalyst outlet side λ is not stoichiometric (λ> 1.0), in step S17, the lean time tl0 is shortened, the lean target λl0 is decreased for the catalyst inlet side λ, and the rich target λr0 is decreased for the catalyst inlet side λ. Any one or combination of the above is performed to reduce the catalyst outlet side λ, and then the process returns to step S13. When the catalyst outlet side λ is stoichiometric (λ ≦ 1.0), the process returns to step S13.

  Then, intermittent pulse control that repeats the desulfurization rich side control in step S13 and the desulfurization lean side control in step S15 is performed until the desulfurization time ts exceeds the predetermined desulfurization completion time ts0. When the desulfurization time ts exceeds the predetermined desulfurization completion time ts0 in step S14, desulfurization completion is performed in step S18 as desulfurization completion, and the process returns.

  In order to see the change of the hydrogen sulfide discharge ratio due to the difference in desulfurization control, one example of continuous rich control and four examples of intermittent pulse control of the present invention are performed, and the results are shown in FIGS.

  Before any desulfurization control, the DPF 11b is regenerated to increase the exhaust gas temperature Tgin and the catalyst temperature due to PM combustion.

In the continuous rich control of the conventional example, the excess air ratio on the inlet side of the catalyst indicated by λ is set to a target air-fuel ratio of 0.9 and constant during the desulfurization control for 3 minutes. In this desulfurization control, it was found that the desulfurization flow rate was 0.26 g, the average hydrogen sulfide ratio was 89%, and most of the hydrogen sulfide (H ₂ S) was discharged. Note that the oxygen concentration on the downstream side of the catalyst indicated by “O ₂ ” in the figure is initially small but gradually increases.

  In the intermittent pulse control of the embodiment of the present invention, the target air-fuel ratio is set to 0.9 in terms of excess air ratio in the case of rich air-fuel ratio, and 1.1 in terms of excess air ratio in the case of lean air-fuel ratio. Then, the rich control time (R) and the lean control time (L) of the intermittent pulse control are expressed as (R / L = 5 s / 2 s) in the embodiment (1) and (R / L = 5 s / in the embodiment (2). 3s), (R / L = 4s / 3s) in Example (3), and (R / L = 3s / 3s) in Example (4).

As a result, the deflow rate and the average hydrogen sulfide ratio indicated by “H ₂ S” in the figure are 0.32 g and 74% in Example (1), 0.31 g and 66% in Example (2), respectively. In Example (3), it was 0.30 g and 50%, and in Example (4), it was 0.06 g and 34%. Further, the oxygen concentration on the downstream side of the catalyst indicated by “O ₂ ” in the figure is also as shown in FIGS.

Of the results obtained in FIGS. 4 and 5, the desulfurization amount S per purge and the average hydrogen sulfide ratio H ₂ S are collectively shown in FIG. According to FIG. 6, in this embodiment, the embodiment (3) has a large amount of desulfurization S and a low average hydrogen sulfide ratio H ₂ S, which is the optimum control condition.

It is a figure which shows the structure of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which shows typically the desulfurization control of embodiment which concerns on this invention. It is a figure which shows the example of the desulfurization control flow of embodiment which concerns on this invention. It is a figure which shows the time series of desulfurization control of continuous rich control and intermittent pulse control (1). It is a figure which shows the time series of desulfurization control of intermittent pulse control (2)-(4). It is a figure which shows the desulfurization amount and average hydrogen sulfide ratio of the experimental result of desulfurization control of continuous rich control and intermittent pulse control (1)-(4).

Explanation of symbols

E engine 1 exhaust gas purification system 2 intake passage 4 exhaust passage 5 EGR passage 6 EGR valve 8 fuel injection valve 9 intake throttle valve (intake throttle valve)
10 Exhaust gas purification device 11a Oxidation catalyst 11b DPF
11c NOx storage reduction catalyst 12 HC supply valve 13 Upstream air excess rate sensor (upstream λ sensor)
14 Downstream oxygen concentration sensor (downstream O ₂ sensor)
15 Upstream temperature sensor 16 Downstream temperature sensor

Claims (6)

  In the exhaust gas purification system comprising a NOx occlusion reduction type catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces NOx that has been occluded when the exhaust is rich, In the desulfurization control for recovering sulfur poisoning of the NOx occlusion reduction catalyst, the control for making the air-fuel ratio on the inlet side of the NOx occlusion reduction catalyst rich and on the lean side are alternately repeated, In the control to bring the target air-fuel ratio on the inlet side to the rich side, 0.85 to 0.95 in terms of excess air ratio, and in the control to make the lean side, 1.05 to 0.55 in terms of excess air ratio. 1. A desulfurization control method for an exhaust gas purification system, wherein 1.15.   2. The desulfurization of an exhaust gas purification system according to claim 1, wherein the target air-fuel ratio on the inlet side is feedback-controlled so that the air-fuel ratio on the outlet side of the NOx storage reduction catalyst becomes a stoichiometric state during the desulfurization control. Control method.   3. The time ratio between the rich side control and the lean side control that is repeated in the desulfurization control is rich side control: lean side control = 5: 2 to 4: 3. Control method for exhaust gas purification system in Japan.   The NOx occlusion reduction catalyst that occludes NOx when the air-fuel ratio of the exhaust gas is lean and releases and reduces the NOx occluded when it is rich, and the sulfur coverage of the NOx occlusion reduction catalyst In the exhaust gas purification system provided with a catalyst regeneration control device that performs desulfurization control for recovering poison, the desulfurization control means is configured to control the lean side of the air-fuel ratio on the inlet side of the NOx occlusion reduction type catalyst and lean And the control to make the desulfurization control means set the target air-fuel ratio on the inlet side to the rich side is 0.85 to 0.95 in terms of excess air ratio, In the control to the lean side, the exhaust gas purification system is set to 1.05 to 1.15 in terms of excess air ratio.   The desulfurization control means feedback-controls the target air-fuel ratio on the inlet side so that the air-fuel ratio on the outlet side of the NOx storage reduction catalyst becomes a stoichiometric state during the desulfurization control. Exhaust gas purification system. The desulfurization control means repeats the desulfurization control so that the time ratio between the rich side control and the lean side control is rich side control: lean side control = 5: 2 to 4: 3. Item 6. The exhaust gas purification system according to Item 4 or 5.

Images